Retrieval of a single complete copy from multiple stored copies of information

ABSTRACT

A method and apparatus for retrieving a single complete copy from multiple stored copies is provided. Information of each of the multiple stored copies is contained in a different set of sectors on disc surfaces in a disc storage system. Each different set of sectors can include at least one defective sector from which information is not recoverable. One copy from the multiple stored copies from which information is recoverable is selected. Defective sectors in the selected copy are identified. Replacement sectors are located from the multiple stored copies other than the selected copy. Information from the selected copy is merged with information from the replacement sectors to form the single complete copy.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationNo. 60/222,990, filed Aug. 4, 2000, and entitled “A ROBUST RESERVEDCYLINDER INFORMATION RETRIEVAL METHOD”.

FIELD OF THE INVENTION

The present invention relates to disc storage systems and, inparticular, to a method and apparatus for retrieval of a single completecopy from multiple stored copies of information.

BACKGROUND OF THE INVENTION

In data storage systems such as disc drives, information is stored upona surface of a medium such as a storage disc in a set of concentriccircular patterns called tracks. Typically, storage discs of a discdrive are stacked and mounted for rotation together on a single spindle.Each side of each disc in the disc stack has a surface which is usuallyused to store information. Each surface of a disc in the disc stack isusually exposed to at least one head responsible for reading and writinginformation on that particular surface. Typically, all the heads whichare mounted on an actuator arm move in tandem radially over the surfacesof the disc so that they are all at the same approximate disc radius atthe same time.

In order to read and write data from the correct location in the discstack, the data storage areas in the disc stack are identified by acylinder address, head address and sector address. A cylinder identifiesa set of specific tracks on the disc surfaces in the disc stack whichlie at equal radii and are, in general, simultaneously accessible by thecollection of heads. The head address identifies which head can read thedata and therefore identifies the disc that the data is recorded on.Each track within a cylinder is further divided into sectors for storingdata and servo information.

Most cylinders are available for read/write access by the host computer.However, drive unique information, for example, the drive defect table(table that contains remap information to circumvent defective sectorsformed at the time the disc was manufactured) is stored in reservedcylinders which are not normally accessible by the host computer.Typically, multiple copies of drive unique information are stored inreserved cylinders. Additionally, multiple copies of user data aresometimes stored on user accessible cylinders for fast access and databackup purposes.

Each of the multiple copies is stored on a different set of sectors.Defects can arise in any of these sectors at various times during thelifetime of the storage system (grown defects). Grown defects include,for example, invading foreign particles which become embedded onto thesurface of the disc, or external shocks to the storage system which cancause the transducer to nick the surface of the disc. Defective sectorspresent within a group of sectors storing any of the multiple copies ofinformation pose either temporary or permanent problems in retrieving anentire copy.

In typical prior art systems, when retrieval of a first copy fails dueto a defective sector being encountered, a second attempt is made toretrieve a second copy. If the second attempt fails, a third attempt ismade to retrieve a third copy and so on. This process is repeated until,either a complete copy is retrieved or until a recovery attempt on thelast available copy has failed. In this retrieval method, any portion ofinformation contained on a defective sector in each copy is sufficientto prevent the retrieval of a complete copy of information. Therefore,at least one copy of information should be contained on a set ofcompletely error-free sectors for the prior art retrieval method towork. Consequently, data can be lost, or in some cases, where criticalinformation like the defect table cannot be retrieved, the drive couldprevent the user from accessing any data stored on the discs.

In general, all data backup systems include redundant copies ofinformation. One such system is the redundant array of inexpensive discs(RAID) system. In typical RAID systems, multiple copies of informationare stored in data arrays with each copy stored on different drive. Insome models of RAID systems, when a failure occurs in an array,subsequent read and write operations are directed to a surviving drive.A replacement drive is then rebuilt using data from the surviving drive.Thus, a surviving drive containing error-free data is needed for thistechnique.

Other models of RAID systems use parity information for data recovery.This technique involves regenerating missing data by determining theappropriate value of each missing bit. Typically, parity information isdistributed among all drives in the array, instead of a dedicated paritydrive to prevent loss of all parity information. However, loss of anydrive still reduces the availability of both data and parity informationuntil the failed drive is regenerated from a surviving drive.

The present invention addresses these problems, and offers otheradvantages over the prior art.

SUMMARY OF THE INVENTION

The present invention relates to disc storage systems capable of mergingrecoverable sectors (error free sectors) from more than one stored copyof information to form a single complete copy to solve theabove-mentioned problems.

A method and apparatus for retrieving a single complete copy frommultiple stored copies is provided. Information of each of the multiplestored copies is contained in a different set of sectors on discsurfaces in a disc storage system. Each different set of sectorsincludes at least one defective sector from which information is notrecoverable. One copy from the multiple stored copies from whichinformation is recoverable is selected. Defective sectors in theselected copy are identified. Replacement sectors are located from themultiple stored copies other than the selected copy. Information fromthe selected copy is merged with information from the replacementsectors to form the single complete copy.

These and various other features as well as advantages whichcharacterize the present invention will be apparent upon reading of thefollowing detailed description and review of the associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a head-disc assembly (HDA) with whichthe present invention is useful.

FIG. 2 is a simplified block diagram of a disc drive storage system inaccordance with the present invention.

FIG. 3 illustrates an example of a prior art method of attempting toretrieve a single copy of information from multiple copies of a defecttable.

FIG. 4 illustrates retrieval of a single copy of information frommultiple copies of a defect table in accordance with an embodiment ofthe present invention.

FIG. 5 illustrates retrieval of a single copy of information frommultiple copies of a defect table in accordance with another embodimentof the present invention.

FIG. 6 illustrates retrieval of a single copy of information frommultiple copies of a defect table in accordance with another embodimentof the present invention.

FIG. 7 shows multiple copies of all information contained on one discsurface.

FIG. 8 shows individual copies of the multiple copies distributed ondifferent disc surfaces.

FIG. 9 shows sectors of individual copies of the multiple copies in aninterleaved form.

FIG. 10 shows a flow diagram of a method of retrieving a single completecopy from multiple stored copies in accordance with the presentinvention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Referring now to FIG. 1, a perspective view of a disc drive, head discassembly (HDA) 100 with which the present invention is useful is shown.The same reference numerals are used in the various figures to representthe same or similar elements. HDA 100 includes a housing with a base 102and a top cover (not shown). HDA further includes a disc stack 106,which is mounted on a spindle motor (not shown) by a disc clamp 108.Disc stack 106 includes a plurality of individual discs which aremounted for co-rotation about central axis 109.

Each disc surface has an associated slider 110 which is mounted in HDA100 and carries a read/write head for communication with the discsurface. In the example shown in FIG. 1, sliders 110 are supported bysuspensions 112 which are in turn supported by track accessing arms 114of an actuator 116. The actuator shown in FIG. 1 is of the type known asa rotary moving coil actuator and includes a voice coil motor (VCM),shown generally at 118. Other types of actuators can be used, such aslinear actuators.

Voice coil motor 118 rotates actuator 116 with its attached sliders 110about a pivot shaft 120 to position sliders 110 over a desired datatrack along a path 122 between a disc inner diameter 124 and a discouter diameter 126. Voice coil motor 118 operates under the control of aclosed-loop servo controller within internal circuitry 128 based onposition information, which is stored on one or more of the discsurfaces within dedicated servo fields. The servo fields can beinterleaved with data sectors on each disc surface or can be located ona single disc surface that is dedicated to storing servo information. Asslider 110 passes over the servo fields, the read/write head generates areadback signal that identifies the location of the head relative to thecenter line of the desired track. Based on this location, actuator 116moves suspension 112 to adjust the head's position so that it movestoward the desired position. Once the transducing head is appropriatelypositioned, servo controller 128 then executes a desired read or writeoperation.

Referring now to FIG. 2, a simplified block diagram of a disc drivestorage system 200 in accordance with the present invention is shown.For simplification, only one disc 202 of disc stack 106 (of FIG. 1) isshown. Spindle 204 connects disc 202 to spindle drive motor 206 whichrotates disc 202 at high speed. Slider 110 that carries the read/writehead is connected to actuator 116 through support arm 208. Controller128 directs the action of actuator 116 which moves support arm 208. Hostcomputer 210 is in communication with controller 128 which is adapted toreceive commands from host 210 and operate the disc drive in accordancewith these commands. When a read operation is sent from the host 210 tocontroller 128, the controller converts a logical block address receivedfrom the host 210 to a physical block address (PBA). Based on the PBA, aseek is performed and data is read from the disc into disc drive buffermemory 212. Typically, when a read error occurs, an error correctionalgorithm that attempts to identify and correct the error at thedefective sector is applied by controller 128.

Critical information, such as the drive defect table, is stored on thedisc 202 in multiple copies in reserved cylinders. One copy of the drivedefect table is retrieved from the disc 202 into buffer memory when thedisc 202 is booted up. Multiple copies of the drive defect table arestored on the disc 202 to ensure that at least one complete copy can beretrieved from the disc 202. Additionally, multiple copies of user dataare sometimes stored on user accessible cylinders for fast access anddata backup purposes. Although storing multiple copies of criticalinformation increases the probability of recovering a complete copy, insome cases portions of information from each of the multiple copies maybe contained on one or more defective sectors. The following is anexample of a condition in which none of the copies are individuallycompletely recoverable.

Referring now to FIG. 3, an example of a prior art method of attemptingto retrieve a single copy of defect table information from multiplecopies is shown. Four copies of defect table information (302, 304, 306and 308) with each having information stored over eight sectors with atleast one of the eight sectors of each copy being defective are shown.Retrieval begins with an attempt to recover first copy 302. Sectors (0),(1) and (2) are successfully recovered, but an attempt to recoverinformation from defective sector (3) of first copy 302 fails. As aresult of this failed recovery attempt on sector (3) of the copy 302,the recovery attempt on copy 302 is terminated and a recovery attempt onsecond copy 304 is initiated. The first four sectors (0-3) of secondcopy 304 are error free and are therefore successfully recovered.However, sector (4) of second copy 304 is defective and the attempt torecover second copy 304 is terminated when defective sector (4) isencountered. An attempt to recover third copy 306 is then initiated.Sectors (0-4), which are error free, are successfully recovered, but therecovery attempt on third copy 306 is terminated when defective sector(5) is encountered. Finally, an attempt is made to recover fourth copy308. Sectors (0-1) of fourth copy 308 are successfully recovered, but anerror is encountered at sector (2) of fourth copy 308. After this failedread, the retrieval attempt is terminated and no further recoveryattempts are made. Consequently, the defect table cannot be retrieved,thereby making the drive inaccessible.

Referring now to FIG. 4, a re-illustration of the example shown in FIG.3 using a new recovery method in accordance with an embodiment of thepresent invention is shown. The retrieval attempt begins with a readattempt 402 on first copy 302. On encountering defective sector (3), thenumber of error free sectors retrieved (0-2) is recorded. Next a readattempt 404 to read second copy 304 is initiated. Four error freesectors are read successfully from second copy 304 before a defectivesector (4) is encountered. Again, the number of error free sectors read(0-3) are recorded followed by a third read attempt 406 to read thirdcopy 306. Encountering the defective sector (5), the error-free sectors(0-4) are recorded. The process is repeated until an attempt to read allcopies available is completed. In this example, a read attempt 408 isperformed on fourth and last copy 308. After encountering defectivesector (2) in read attempt 408, the number of error free sectors (0-1)are again recorded. The error free sectors from the copy with thelongest sequence of error free sectors are then loaded into memory. Inthis example the sequence of five error free sectors (0-4) from copy 306are the longest sequence of error free sectors and are therefore loadedinto memory.

A second set of read attempts is then carried out to find error freesectors (5-7) to form a complete copy of the defect table. Thisoperation begins with a second read attempt 412 on first copy 302 whichis carried out starting at first subsequent error free sector (5).Sector (5) is retrieved successfully, but an error is encountered duringthe retrieval of sector (6) of first copy 302. Next, a second readattempt 414 on sectors (5-7) of second copy 304 is carried out. Thisattempt results in successful recovery of error free sectors (5-7) ofsecond copy 304. Therefore, the retrieval process is terminated as alleight sectors have been successfully recovered to form a single completecopy. This retrieval method has enabled a copy of the defect table to beretrieved by merging information from the error free sectors fromdifferent stored copies when none of the copies are individuallyrecoverable.

Referring now to FIG. 5, a recovery method in accordance with anotherembodiment of the present invention is shown. The retrieval attemptbegins with a read attempt 502 on the first copy 302. After sectors(0-2) are retrieved, defective sector (3) is encountered. Onencountering defective sector (3), read attempt 502 is terminated anddefective sector (3) is recorded. Next a read attempt 504 to read secondcopy 304 is initiated. Read attempt 504 starts from the recordeddefective sector (3) in read attempt 502. Sector (3) is recoveredsuccessfully, but an error is encountered during the retrieval of sector(4). On encountering defective sector (4), read attempt 504 isterminated and defective sector (4) is recorded. Read attempt 506,performed on copy 306, starts at last recorded defective sector (4).Sector (4) is recovered successfully, but an error is encountered duringthe retrieval of sector (5), which is recorded. Read attempt 508,performed on copy 308, starts at last recorded defective sector (5).After sector (5) is recovered successfully, an error is encounteredduring the retrieval of sector (6), which is recorded. Read attempt 508,performed on copy 308, is the last of a first set of read attemptsperformed on all copies of information. Since a single complete copy hasnot yet been successfully recovered, a second set of read attempts iscarried out starting from sector (6) of copy 302. Sector (6) is the lastrecorded defective sector at the end of the first set of read attempts.Read attempt 512, performed on copy 302, starts at last recordeddefective sector (6). Since sector (6) of copy 302 is defective, readattempt 512 is terminated and read attempt 514 is performed on copy 304beginning at sector 6. This attempt results in successful recovery ofsectors (6-7) of copy 304. Thus, the retrieval process is terminated asall eight sectors have been successfully recovered to form a singlecomplete copy.

An unsuccessful termination of the retrieval process occurs if adefective sector encountered during a current read attempt on aparticular copy is the same as a recorded defective sector from aprevious read attempt on the same copy. In the above example, readattempt 502 on copy 302 results in sector (3) being the recordeddefective sector. If, for example, sector (3) is also found defective oncopies 304, 306 and 308 during read attempts 504, 506 and 508,respectively, then read attempt 512 will also begin on sector (3) ofcopy 302. If sector (3) is found defective during read attempt 512, theretrieval process terminates unsuccessfully.

Referring now to FIG. 6, a recovery method in accordance with anotherembodiment of the present invention is shown. The retrieval attemptincludes retrieving all error free sectors (0, 1, 2, 5, and 7) from copy302 with the help of read attempts 602, 604 and 606, respectively. Alldefective sectors (3, 4 and 6) are recorded. Read attempt 612, performedon copy 304, begins at first recorded defective sector (3) which issuccessfully recovered, but terminates at defective sector (4). Readattempt 614, also performed on copy 304, begins at sector (6), which issuccessfully recovered. Final read attempt 620, performed on copy 306,begins at sector (4) which is successfully read. The retrieval processis terminated as all eight sectors have been successfully recovered toform a single complete copy. In general, the data recovery method ofFIG. 6 includes retrieving information from all error free sectors of afirst stored copy of data (such as 302), and recording all defectivesectors of the first stored copy (such as 302). Next, a read attempt forrecovery of defective sectors is carried out on a second stored copy(such as 304). The process is repeated on subsequent copies (such as306, 308) until all defective sectors are recovered. The recoveryattempt terminates unsuccessfully if a defective sector is encounteredon a last stored copy of data (such as 308).

Although the above examples relate to recovery of a defect table, ingeneral, the same retrieval process can be applied for recovery of asingle complete copy from any set of multiple stored copies.

FIGS. 7-9 illustrate different forms of storing multiple copies ofinformation. FIG. 7 illustrates a form of storage where all multiplecopies 702 are contained on one disc surface 700. In FIG. 8 a form ofstorage is shown where individual copies of the multiple copies 702 areeach stored on a different disc surface of disc stack 800. FIG. 9illustrates a sector interleave method of storage where consecutivelynumbered sectors are not laid next to each other in multiple copies 702on disc surface 900. The new scheme for merging error free sectors frommultiple stored copies can be applied in all the above forms of storingmultiple copies of information.

Referring now to FIG. 10, a flow diagram illustrating a method ofmerging information from the error free sectors from different storedcopies to form a single complete copy in accordance with the presentinvention is shown. At block 1010 of FIG. 10, one copy having a longestsequence of error free sectors from which information is recoverablefrom multiple stored copies is selected. In block 1020, defectivesectors in the copy having the longest sequence of error free sectorsare identified. At block 1030, replacement sectors from multiple storedcopies other than the copy having a longest sequence of error freesectors is located. At block 1040, information from the longest sequenceof error free sectors is merged with information from the locatedreplacement sectors to form a single complete copy.

The present invention can be summarized in reference to the figures,which illustrate (1) HDA 100 and components thereof, (2) block diagramof a disc drive storage system 200, and (3) examples of the new methodof merging error free sectors from multiple copies. The method includesselecting one copy (302) from which information is recoverable from themultiple stored copies (302-308). Defective sectors are then identifiedin the selected copy (302). Replacement sectors from the multiple storedcopies other than the selected copy are located to replace defectivesectors. Information from the selected copy (302) is merged withinformation from the replacement sectors located to form the singlecomplete copy.

In some embodiments, selecting one copy from multiple stored copiesincludes selecting a copy with the longest sequence of error freesectors (306). In some embodiments, selecting one copy with the longestsequence of error free sectors (306) includes sequentially reading eachsector of the set of sectors from each one of the multiple stored copies(302-308). A number of error free sectors read before a first defectivesector is encountered in each stored copy (402-408) is recorded. Thecopy having the longest sequence of error free sectors (306) from therecorded number of error free sectors corresponding to each copy of themultiple stored copies is then identified.

In some embodiments, locating replacement sectors is performed byselectively reading the set of sectors of at least one of the multiplestored copies other than the copy having a longest sequence of errorfree sectors (306). The selective reading is restricted to sectorswithin the set of sectors that can replace defective sectors.

In some embodiments of the present invention, information from the errorfree sectors is merged in a buffer memory 212. In some embodiments, themultiple stored copies 702 are all contained on one disc surface 700. Insome embodiments, individual copies of the multiple stored copies 702are distributed on different discs 800. In some embodiments, individualcopies of the multiple stored copies 702 are interleaved with eachother.

Embodiments of the present invention also include a disc drive storagesystem with at least one rotatable disc 202 having a disc surfaceincluding multiple stored copies (302-308) of information. Informationof each of the multiple stored copies is contained in a different set ofsectors on the disc surface. Each different set of sectors includes atleast one defective sector from which information is not recoverable.Transducer head 110 is configured to read from the disc surface.Controller 128 selects one copy (302) from which information isrecoverable from the multiple stored copies (302-308). Controller 128also identifies defective sectors in the selected copy (302).Replacement sectors from the multiple stored copies other than theselected copy are located by controller 128. Finally, controller 128merges information from the selected sectors with information from thereplacement sectors located to form a single complete copy.

In some embodiments, controller 128 selects one copy of the multiplestored copies having a longest sequence of error free sectors (306). Insome embodiments, controller 128 sequentially reads each sector of theset of sectors from each of the multiple stored copies. During thesequential read, controller 128 records a number of error free sectorsread before a first defective sector is encountered in each copy.Controller 128 then identifies the copy having the longest sequence oferror free sectors from the recorded number of error free sectorscorresponding to each copy of the multiple stored copies.

In some embodiments, controller 128 selectively reads the set of sectorsof at least one the multiple stored copies other than the copy having alongest sequence of error free sectors to locate replacement sectors. Insome embodiments of the present invention, information from the errorfree sectors is merged in a buffer memory 212 by controller 128.

It is to be understood that even though numerous characteristics andadvantages of various embodiments of the invention have been set forthin the foregoing description, together with details of the structure andfunction of various embodiments of the invention, this disclosure isillustrative only, and changes may be made in detail, especially inmatters of structure and arrangement of parts within the principles ofthe present invention to the full extent indicated by the broad generalmeaning of the terms in which the appended claims are expressed. Forexample, the particular elements may vary depending on the particularapplication for the disc retrieval operation while maintainingsubstantially the same functionality without departing from the scopeand spirit of the present invention. Also, although the preferredembodiment discloses determining the longest sequence of error freesectors beginning at sector (0), equivalent alternative embodiments arecontemplated wherein the longest sequence of error free sectors isdetermined beginning at any desired sector or regardless of where thesequence begins. In addition, although the preferred embodimentdescribed herein is directed to retrieving a single copy complete copyof information from multiple copies of the same information for a discstorage system, it will be appreciated by those skilled in the art thatthe teachings of the present invention can be applied to systems, likemagnetic, optical or other storage system techniques, without departingfrom the scope and spirit of the present invention.

1. A method of retrieving a complete copy of data from a plurality ofstored copies of the data, the plurality of stored copies contained in adifferent set of storage locations in a data storage system, the methodcomprising: (a) selecting one of the copies from the plurality of storedcopies; (b) identifying defective storage locations in the selectedcopy; (c) locating replacement storage locations from the plurality ofstored copies other than the selected copy; and (d) merging storagelocations from the selected copy with replacement storage locationsdefining the complete copy of substantially error free storagelocations.
 2. The method of claim 1 wherein step (a) includes selectingone of the copies from the plurality of stored copies having a longestsequence of error free storage locations.
 3. The method of claim 1wherein each of the plurality of stored copies of data comprises atleast one defective storage location from which data is not recoverable.4. The method of claim 1, wherein the selecting one copy step (a)comprises: (a)(1) sequentially reading each storage location of the setof storage locations from each of the plurality of stored copies; (a)(2)recording a number of error free storage locations read before a firstdefective storage location is encountered when each of the plurality ofcopies is sequentially read in accordance with step (a)(1); and (a)(3)identifying one copy having a longest sequence of error free storagelocations from the recorded number of error free storage locationscorresponding to each copy of the plurality of stored copies.
 5. Themethod of claim 1, wherein the locating replacement storage locationsstep (c) is performed by locating the set of storage locations of atleast one of the plurality of stored copies other than the selectedcopy, wherein the locating is restricted to reading storage locationswithin the set of storage locations that can replace defective storagelocations identified in step (b).
 6. The method of claim 1, wherein themerging storage locations step (d) is performed in a buffer memory. 7.The method of claim 1, wherein the plurality of stored copies is allcontained on one disc surface.
 8. The method of claim 1, whereinindividual copies of the plurality of stored copies are distributed ondifferent disc surfaces.
 9. The method of claim 1, wherein individualcopies of the plurality of stored copies are interleaved.
 10. A discdrive storage system implementing the method of claim
 1. 11. A datastorage system, comprising: at least one storage medium having aplurality of stored copies of information, with information of each ofthe plurality of stored copies contained in a different set of storagelocations; and a controller configured to select one copy of theplurality of stored copies, and to identify defective storage locationsin the selected copy, and to locate replacement storage locations fromthe other stored copies, and to merge storage locations from theselected copy with the replacement storage locations defining a completecopy, of substantially error free storage locations, of the storedinformation.
 12. The data storage system of claim 11 wherein thecontroller is further adapted to select one copy of the plurality ofstored copies having a longest sequence of error free storage locations.13. The data storage system of claim 11 wherein each of the plurality ofstored copies of information comprises at least one defective storagelocation from which data is not recoverable.
 14. The data storage systemof claim 11, wherein the controller is further adapted to sequentiallyread each storage location from each of the plurality of stored copies,and to record a number of error free storage locations read before afirst defective storage location is encountered when each of theplurality of stored copies is read, and to identify the copy having alongest sequence of error free storage locations from the recordednumber of error free storage locations corresponding to each copy of theplurality of stored copies.
 15. The data storage system of claim 11,wherein the controller is further adapted to selectively read the set ofstorage locations of at least one of the plurality of stored copiesother than the copy having a longest sequence of error free storagelocations.
 16. The data storage system of claim 11, further comprising abuffer memory to temporarily store the complete copy.
 17. The datastorage system of claim 11, wherein the plurality of stored copies iscontained on one data storage medium.
 18. The data storage system ofclaim 11, wherein individual copies of the plurality of stored copiesare distributed on different data storage media.
 19. The data storagesystem of claim 11, wherein individual copies of the plurality of storedcopies are interleaved.
 20. A data storage system, comprising: at leastone storage medium having a plurality of stored copies of information,with information of each of the plurality of stored copies contained ina different set of storage locations; and a controller means forselecting one copy of the plurality of stored copies from whichinformation is recoverable, and for identifying defective storagelocations in the selected copy, and for locating replacement storagelocations from the other stored copies, and for merging storagelocations from the selected copy with the replacement storage locationsdefining a complete copy, of substantially error free storage locations,of the stored information.